Automated fiber placement machines are widely used to manufacture parts, components and structures from composite materials. The materials used in automated fiber placement are typically composed of longitudinal fibers and resin consolidated into tapes, or thin strips commonly known as “tows.” Individual tapes or tows are manipulated by the fiber placement machine to form a band of material that is deposited “laid” onto a tool. Parts are built up layer by layer, with tapes or tows of composite material, with the angle at which each layer “ply” is laid onto the tool being precisely determined by the fiber placement machine.
The tool that the composite material is laid onto is often quite complex, in that the geometry of the finished part is machined into the tool surface. Depending upon the desired result, the tool may have the form of the outside “Outer Mold Line—OML” or the inside of the part “Inner Mold Line—IML”.
In the past, tools used with automated fiber placement machines have typically taken the form of a rotating mandrel supported by a headstock and tailstock. As the mandrel rotates, while supported by the headstock and tailstock, the automated fiber placement machine precisely lays bands of material onto the rotating mandrel to produce full revolution parts. After the layers of composite material have been laid onto the mandrel by the fiber placement machine, the mandrel, with the composite material wrapped thereupon, is removed from the headstock and tailstock and placed into an autoclave for final curing of the composite structure. After the composite structure has been cured in the autoclave, the mandrel is removed from the finished structural part.
Although the use of rotating mandrel tools with automated fiber placement machines generally works well for cylindrical-shaped components, such as fuselage sections, and nose or tail cones in aircraft manufacture, for example, the use of rotating mandrels does present certain problems for designers and manufactures of composite structures. Because the mandrel must be placed into the autoclave with the composite structure for curing the composite material deposited on the surface of the mandrel, the autoclave is required to heat the mass of the mandrel to a sufficient temperature for curing the part, which greatly increases cycle times and operating cost for the autoclave portion of the manufacturing process. Also, because the composite materials are wrapped sequentially onto the outer surface of the mandrel, it is not possible to form an outer mold line “OML” surface on the composite structure. Additional problems are encountered, when attempting to use rotating mandrels for the formation of parts which are not symmetrical about an axis of rotation, thus creating problems and balancing the mandrel, or parts which have a cross section which is flat, complex, or otherwise constricted in such a manner that it is very difficult to remove the mandrel from the part, after the autoclave cycle.
Furthermore, there are many parts and structures which are not closed surfaces of revolution, but are instead more planar in nature. A typical example of such a part is a wing skin of an aircraft. Although wing skins are generally sculptured surfaces, the curvature involved is relatively gradual and not closed, as is the case for a fuselage part. Although rotating mandrels have sometimes been used, in the past, to form such parts, it has been necessary to provide means and manufacturing processes for essentially cutting the desired planar-shaped part away from the remainder of the wound structure, following the autoclave cycle, resulting in undesirable manufacturing cost and waste of material.
What is needed, therefore, is an improved method and apparatus for forming composite structures with an automated fiber placement machine, and in particular, an improved method and apparatus for forming composite structures having a planar, or other shape which is not amenable to production by prior automated fiber placement manufacture using rotating mandrels. It is further desirable to provide an improved method and apparatus for manufacturing the upper and lower wing skins for an aircraft with an automated fiber placement machine.